Concentrating and collecting optical system using concave toroidal reflectors

ABSTRACT

An electromagnetic radiation source, such as an arc lamp, is located at a point displaced from the optical axis of a concave toroidal reflecting surface. The concave primary reflector focuses the radiation from the source at an off-axis image point that is displaced from the optical axis. The use of a toroidal reflecting surface enhances the collection efficiency into a small target, such as an optical fiber, relative to a spherical reflecting surface by substantially reducing aberrations caused by the off-axis geometry. A second concave reflector is placed opposite to the first reflector to enhance further the total flux collected by a small target. In accordance with one embodiment, the present invention is directed to devices in which the square of the off-axis distance divided by the radius of curvature is equal to or less than the extent of the source of electromagnetic radiation (y 0   2 /r≦s 0 ).

This application is a continuation-in-part of U.S. Ser. No. 08/488,188,filed Jun. 7, 1995, now U.S. Pat. No. 5,836,667 allowed, which was acontinuation of Ser. No. 07/924,198, filed Aug. 3, 1992, now U.S. Pat.No. 5,430,634.

FIELD OF THE INVENTION

This invention relates to systems for collecting and condensingelectromagnetic radiation, particularly a system for providing a highradiance to a small target such as an optical fiber.

BACKGROUND OF THE INVENTION

Conventional collecting and condensing designs for electromagneticradiation emphasize collecting and redirecting the maximum amount oflight from a source of radiation, approximated by a point source. Toproduce a small spot size based on these designs results in a decreasein radiation flux because conventional designs (i.e., the collection andredirection of the maximum amount of light) inherently conflict with thegoal of concentrating the radiation flux into the smallest possible spotsize when the radiation originates from conventional incoherent sources.Thus, images of small spot size may be obtained only by a correspondingdecrease in flux density.

There are two basic optical designs in common use for collecting andcondensing radiation. The first is a system of condenser lenses such asillustrated in FIG. 1. Condenser lenses have several problems whichinclude creation of chromatic and spherical aberrations, relatively highcost, inherently difficult alignment, and large amount of space.Ellipsoidal reflectors as shown in FIG. 2 are also used in prior artsystems. Their problems also include high cost and an unavoidablemagnification of the image (i.e. a reduction in the flux density). Bothof these systems tend to emphasize redirection of the maximum amount offlux from a point source at the expense of the flux density, asdiscussed above.

U.S. Pat. No. 4,757,431, the embodiment of which is incorporated hereinby reference (FIG. 3), describes an improved condensing and collectingsystem employing an off-axis spherical concave reflector which enhancesthe maximum flux illuminating a small target and the amount ofcollectable flux density by a small target. The off-axis sphericalconcave reflector described in this patent has certain disadvantages,namely, astigmatism parallel to the direction of the off-axisdisplacement and the physical limitations inherent in the requirement tominimize this distance. The effect of astigmatism is to decrease theconcentrating efficiency of the system and thereby reduce the fluxcollected at a target. The requirement to minimize the off-axis distancebetween the source and the target (i.e. minimize astigmatic distortion),imposes limitations on the physical dimensions of a source and target ofthe described embodiment. The teachings of the use of a deformablespherical concave reflector does not lead to the use of a toroidalreflector having two perpendicular and unequal radii of curvature.

SUMMARY OF THE INVENTION

The present invention represents an improvement over the systemdisclosed in U.S. Pat. No. 4,757,431 in three ways: (i) it enhances theconcentration and collection of radiation emitted by a point-like sourceof electromagnetic radiation into a small target; (ii) it increases thecollectable flux into a small target, and (iii) it improves thecollection and coupling efficiency between a source of electromagneticradiation and a small target for any “off-axis optical system” asdescribed in U.S. Pat. No. 4,757,431, particularly in the reduction ofthe preferred embodiment into practical systems.

To achieve these and other objectives, the present invention employs asthe primary optical element a concave reflecting surface havingdifferent radii of curvature along two orthogonal axes (i.e. a toroidalreflector), a source of electromagnetic radiation and a target (i.e. anoptical fiber). The source and target are located at similar distanceson opposite sides of the optical axis of the system which is defined asthe optical axis of the concave toroidal reflector (the “off-axisreflector”). For concentrating maximum flux density at the target, aretro-reflector, preferably of toroidal design or alternatively ofspherical design, is located behind the source to reflect and re-focusradiation from and back through the source onto the toroidal reflector.The retro-reflector together with the off-axis toroidal reflector act asa system for maximizing the collectable flux density of radiationconcentrated at the target. The system substantially improves thecollectable radiance at the target in two ways: (i) the toroidal designof the reflectors substantially corrects for astigmatism caused both bythe off-axis geometry and glass-envelope of typical electromagneticradiation sources such as an arc lamp and (ii) the retro reflectorincreases the effective brightness of the radiation source. The maximumoptical efficiency of the system is obtained by optically matching thereflectors and target, while the maximum flux density at the target and,in particular, collectable by an optical fiber as the target, isobtained both by maximizing the system efficiency and optically matchingthe source, reflectors, and target. Whereas prior art teaches the use ofellipsoidal reflectors “on-axis” or deformable spherical concavereflectors “off-axis,” in practice the use of aspheric mirrors isexpensive. A significant advantage of the present system is the use ofvery inexpensive aspheric mirrors, toroids, to concentrate light at atarget in which the collectable flux density at the target isinsensitive to the surface quality of the mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art condenser lens system.

FIG. 2 is a schematic illustration of a prior art ellipsoidal lenssystem.

FIG. 3a is a schematic illustration in the y-z plane of a prior artsystem employing a spherical reflector.

FIG. 3b is a schematic illustration in the x-z plane of a prior artsystem employing a spherical reflector.

FIG. 4a is a schematic illustration in the x-z plane of the presentinvention.

FIG. 4b is a schematic illustration in the y-z plane of the presentinvention.

FIG. 5 is a coordinate system of the embodiment of the presentinvention.

FIG. 6a is a ray diagram illustrating the optimum image locations for aconcave spherical reflector to maximize the concentration and collectionof radiation at a target.

FIG. 6b is a ray diagram illustrating the optimum image locations for aconcave toroidal reflector to maximize the concentration and collectionof radiation at a target. Note that the result of having two radii ofcurvature to compensate for optical aberrations nearly collapses I₁ andI₂ in a theoretical sense into the circle of least confusion. In apractical sense, I₁ and I₂ are at the circle of least confusion (seeFIG. 6a) and the size of the image at the circle of least confusion islarger than that of the source depending on the extent to which thetoroidal design is optimized.

FIG. 7a is a schematic illustration of the optical configuration of FIG.3a expanded to include two off-axis and two secondary retro-reflectors.

FIG. 7b reduces the two secondary retro-reflectors of FIG. 7a to asingle nearly hemispherical reflector having two radii of curavture inorthogonal planes unequal (toroidal) or equal (spherical) depending onthe source.

FIG. 8 is a schematic illustration of the optical configuration in whichthe reflectors and source are assembled and substantially fabricated asone self contained unit.

FIG. 8A schematically shows a detail of a fiber optic target placed neara window in the optical configuration of FIG. 8.

FIG. 9 is an extension of the invention to include four targets coupledto the electromagnetic radiation source by four off-axis toroidalreflectors.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular numbers,dimensions, optical components, etc. in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that the present invention may be practiced inother embodiments that depart from these specific details. In otherinstances, detailed descriptions of well known devices and techniquesare omitted so as not to obscure the description of the presentinvention with unnecessary detail.

A condensing, collecting, and concentrating optical system built inaccordance with this invention consists of three main components (FIG.4). The fourth, a retro-reflector, is optional, but improvesperformance.

(1) Source. An optical point source of electromagnetic radiation. In thecontext of this invention, a point source is any compact source ofelectromagnetic radiation whose angular extent is small and emits fluxinto 4π stearadians. Typically, the linear angular size of such a sourceis no more than 0.1 radian. For example, a typical source may be anelectric arc lamp with an arc gap of approximately 1 mm placed in frontof a concave reflector at a distance of approximately 50 mm. Inpractice, such a source is an extended source. In the preferredembodiment, this is a compact xenon arc lamp with an arc gap≦1 mm and aquartz lamp envelope or ceramic enclosure with a quartz window. However,any source of electromagnetic radiation which is of similar size to orsmaller than the target may be used (e.g. fiber, filament lamp, gasdis-charge lamp, laser, LED, semi-conductor, etc.). The size of theelectromagnetic source here is better defined by the 1/e intensity ofthe intensity contour map which characterizes the brightness (fluxdensity over angular extent) of the source. Brightness is related to thesize of the arc gap and determines the theoretical limit of couplingefficiency. For the specific case of an arc lamp, the contourapproximates axial symmetry and is a complex function of electricalrating, electrode design and composition, gas pressure, arc gap size,and gas composition. For the specific case of an arc lamp having anaspherical curved envelope, the effective relative position andintensity distribution of the source imaged by the reflector undergoesaberration. This is caused by the shape of the envelope whichessentially functions as a lens and requires a compensating opticalelement. Optical compensation can be achieved either by modifying thedesign of the reflector to compensate for the astigmatism caused by theenvelope or by inserting a correcting optic between the off-axisreflector (see below) and the target. Additionally, optical coatings canbe applied to the envelope to minimize Fresnel reflections and therebymaximize collectable radiation at the target or to control and/or filterthe radiation flux.

(2) Reflector. The reflector (off-axis) reflects and focuseselectromagnetic radiation from the source onto the target. Its opticalaxis defines the Z axis of the system, relative to which both the sourceand target are off-axis. In the context of this invention, the reflectoris a portion of a toroidal reflector concave relative to the source, theexact design and placement for which depends on the characteristics ofthe source and the target. In the preferred embodiment of thisinvention, its exact design depends on the characteristics of the sourceand the target. These characteristics are as follows: for the target (i)size, (ii) shape, (iii) off-axis displacement (see below), and (iv) fora fiber optic target, the numerical aperture, diameter, and angle of theproximal end relative to the off axis reflector, defined as the anglebetween the transverse and longitudinal axes of the fiber; for thesource, (i) size and brightness, (ii) effective numerical aperture, and(iii) astigmatism caused by the source envelope or enclosure if present.Optical coatings can be applied to the surface of the reflector toenhance reflection or to control and/or filter, the radiation flux.Additional astigmatic correction can be accomplished with a lens or tiltplate inserted between the reflector and target, with or without opticalor dielectric coatings.

(3) Target. The target is a small object which needs to be irradiated orilluminated with the highest flux density electromagnetic radiationpossible. In the preferred embodiment, it is a single optical fiber witha diameter near 1 mm or smaller. The properties of the optical fiber,diameter and numerical aperture, must be matched to the opticalcharacteristics of the system consisting of source and reflector. Theefficiency of collection and transmission can be enhanced or controlledby adding optical preparations to the input end of the fiber.Alternatively, the target can be a single optical fiber or a grouping ofoptical fibers having similar or dissimilar shapes, sizes, materials,and numerical apertures and arranged either symmetrically orasymmetrically. The end(s) of the fiber(s) is (are) typicallyflat-polished, perpendicular to the longitudinal axis (axes) of thefiber(s); however, the end proximal to the reflector can be polished atan angle (i) to compensate both for the asymmetric image of theelectromagnetic radiation source, such as an arc lamp and forastigmatism introduced by the off-axis geometry and lamp envelope, (ii)to modify the relative numerical aperture of the fiber to the opticalcollection system, and (iii) to adjust for the relative angle of thelongitudinal axis of the proximal end of a fiber optic target relativeto the optical axis of the system.

(4) Retro-reflector. A retro-reflector reflects and re-focuses radiationfrom and back through a source, effectively increasing the brightness ofthe source by overlaying an inverted intensity distribution of radiationonto the original source. In the preferred embodiment of this invention,the retro-reflector is a portion of a toroidal reflector concaverelative to the source. In an alternative embodiment, theretro-reflector is a portion of a spherical reflector. Its exact designsdepend on the shape and size of the source relative to the size of thetarget (and the numerical aperture in the case of a fiber optic target)and the aspheric correction necessitated by the source envelope, if any.Additionally, optical coatings can be applied to the surface of theretro-reflector to enhance reflectivity, or to control, filter, and/orattenuate radiation flux.

FIGS. 4a & 4 b illustrate an idealized concentrating and collectingsystem according to the present invention. On opposite sides of theoptical axis O of the system are a source S_(o) and target T eachdisplaced a distance y_(o) from the optical axis, defined by the centerof curvature and optical axis of toroidal reflector M₁ (off-axisreflector). (The optical axis of a toroidal reflector is defined as thenormal to the perpendicular intersection of the radii of curvature.)Also, a retro-reflector M₂ is located behind the source s_(o) with thesource at a distance approximately equal to its radius of curvature.Although the preferred embodiment includes this retro-reflector formaximal concentration of radiant flux density, it is not essential forcondensing, concentrating, and collecting radiation at the target.

As shown in FIG. 4, the off-axis displacement, y_(o), is equal for asource S_(o) and target T. In the reduction to practice of the presentinvention, the off-axis displacement of the source may be different fromthat of the target. For the latter, the effective optical axis of thesystem will lie between the target and the source and may be differentfrom the optical axis of the reflector. The exact location of effectivesystem optical axis in this case will depend on the numerical apertureof the target and the effective numerical aperture of the reflector.When the optical axis of the off-axis reflector is not an exactdescription of the system optical axis, the effective optical axis ofthe system is determined from a proper matching of the numericalaperture of the source to the effective numerical aperture of thereflector and the numerical aperture of the target. The effectivenumerical aperture of the reflector will differ from the theoreticalnumerical aperture if that portion of the reflector actually used tocondense and concentrate light within the acceptance angle of the targetis smaller than the full aperture, A₁, in FIG. 4. For systems in whichthe numerical aperture of the target is less than that of the off-axisreflector, the effective numerical aperture of the reflector will beless than its theoretical numerical aperture.

It will be observed that the geometry of the system illustrated in FIGS.4a & 4 b is quite similar to that which is disclosed in U.S. Pat. No.4,757,431, FIGS. 3a & 3 b, shown here for comparison. As explained inthat patent, the use of a spherical reflector imposes the restrictionsthat the square of the off-axis distance (y_(o) ²) divided by the radiusof curvature of the off-axis reflector (r) be less than the extent ofthe source (S_(o)). As discussed below, this restriction is eased by theenhancements of the present invention.

Whereas the above-cited patent teaches (i) that the source relative tothe spherical concave mirror should be placed at a distance along the zaxis equal to the radius of curvature of the mirror and a distance,y_(o), off-axis, such that (y_(o) ²)/r<S_(o) and (ii) that the optimumlocation for a target is then the image point defined as the circle ofleast confusion, further analysis reveals that positioning the target atthis location is not necessarily optimum as defined in U.S. Pat. No.4,757,431. Its exact location depends on the characteristics of thesource, of the reflectors, and/or of the transmissive optic(s) placedbetween the target and the source. It also depends on the target and, inparticular, for an optical fiber, on its shape, size, numerical aperture(NA), and cross sectional angle relative to the optical axis at theproximal end relvative to the off-axis reflector. The present invention,therefore, is an optical system that increases and enhances theconcentration and collection of radiant flux at a target. It alsoincreases and enhances the degree of illumination of the target. In thecase of a multimode optical fiber as the target, the fiber may act as anactive element that randomizes and scrambles transmitted radiant fluxthereby eliminating optical aberrations and optical memory. FIG. 5further illustrates the coordinate system of one embodiment of thepresent invention.

Because the optical system of the present invention can be constructedwith many variations in sources, targets, and optical components, thelocation of maximum collectable flux for a given target is defined asthe location of maximum flux density for the specific set of componentsof the system and may or may not coincide with the location of maximumtotal flux density, total flux , or image point (circle of leastconfusion). For targets placed at the image point, the present inventionprovides an optical imaging system of increased radiant flux densitycompared to what is achievable with prior art. Nevertheless, this systemmay not be optimized to provide the maximum theoretical collectionefficiency. The optimum location for the placement of a target in thepresent invention will depend on the characteristics of the target andcan be classified as follows.

Case 1: For targets placed at the image point (circle of leastconfusion) which are of similar size to or larger than that of thesource, the system has approximately unit magnification. In this casethe system is typically optimized if a fiber optic target has anumerical aperture equal to or larger than that of the off-axis mirror.

Case 2: For targets smaller than the source or for fiber optic targetshaving a smaller numerical aperture than that of the off-axis mirror ofcase 1, there exist toroids specific for a given source and a targetthat optimize the collectable flux density at the target which may bedifferent than case 1. Hence, for a target described by case 2, there isa corresponding optimized toroid for a given source. These systems ofsources and targets of unmatched size, having specifically optimizedoff-axis toroidal reflectors, image at a magnification approximatelyequal to unity as in case 1 and the target is placed at the image point.

Case 3: For practical systems involving a given source and a toroidalreflector optimized for a target of specified characteristics (e.g.,diameter, shape, numerical aperture for a fiber optic target asdiscussed in cases 1 and 2), use of such an optimized system withtargets having sizes or numerical apertures other than those of theoptimized target may require different positioning of the target and thereflector relative to the source. In case 3, the system deviates fromunit magnification in that the toroidal reflector typically must betranslated along the z-axis and positioned relative to the source at adistance so as to optimize the positioning of that portion of thereflector-surface which concentrates the maximum flux density within theangle of acceptance of the target. Relative to cases 1 and 2, thelocation of the target may differ substantially and the effectivenumerical aperture of the reflector is matched to the numerical apertureof the target. The effective optical axis of the system may also differfrom the idealized geometry of FIG. 4.

In systems characterized by case 3, there may exist a locus of pointshaving similar collectable flux densities for a given target dependingon the characteristics of the source. For arc sources and other similarextended sources, that portion of the intensity contour collectable by afiber optic target will vary with target size and with numericalaperture of both target and off-axis mirror. Hence, that portion of thesource actually imaged or collected at the target varies. For smalltargets there may exist more than one part of the intensity contour thatproduces the same collectable flux density at the target, enabling thetarget to be placed at a locus of points to achieve similar collectableflux densities. Thus for case 3, the system is said to concentraterather than image flux density from the source at the target. In thiscase for which a locus of points of similar flux densities exist for agiven target, the size of the target will always be smaller than that ofthe source and the source will have an intensity contour that will varyover its nominal size.

In the present invention and in U.S. Pat. No. 4,757,431, the degradationof the radiant flux of the source at the focal point, image point, orlocation of the target is primarily caused by astigmatism produced inthe y-direction by the off-axis geometry. Secondarily, for a sourcecontaining a glass envelope, such as an arc lamp, astigmatism is causedby the aspherical shape of the glass envelope itself. In U.S. Pat. No.4,757,431 the deficiency of the spherical reflector is that theprojection of the rays onto the y-z plane converge closer to thereflector than do the projection of the rays onto the x-z plane. Thecurrent invention improves on the teachings of this patent in thesubstitution of a toroidal surface having its longer radius of curvaturealong the y-axis and shorter radius of curvature along the x-axis. Thedifference in radii causes the convergence of rays in the y-z plane tobe repositioned to coincide with that in the x-z plane. Thissubstitution reduces the size of the focal point by reducing the totalsystem astigmatism, thereby both increasing the concentrating power ofthe optical system and enhancing the collectable radiant flux at thetarget. In the specific case of a target being placed at the circle ofleast confusion, a toroidal reflector substantially reduces the size ofthe image. For example, comparison of the maximum attainable flux,collected by a 1 mm diameter optical fiber from a xenon arc lamp havinga nominal 1 mm arc gap, from a spherical reflector and a toroidalreflector, each having the same NA and effective radius, has shown thata toroidal reflector can increase the maximum collectable flux bygreater than 40%.

Another advantage of a toroidal over a spherical reflector is itsadaptability in reducing and compensating for aberrations in off-axisgeometries when non-ideal point sources (e.g., extended sources withaspherical glass envelopes) are used. By rotating the toroidal reflectoraround the z-axis it is possible to compensate for any of theseaberrations and thereby adjust for the practical variations inmanufacturing tolerances of components in the optical system. Thisrotation adjusts the effective focal lengths defined by the radii ofcurvature along the x-z and y-z planes and thereby concentrates theradiation flux density to a maximum extent. Thus rotation of thetoroidal reflector optimizes the flux density at the target by adjustingthe radii of curvature to compensate for the particular aberrations inthe system.

A practical improvement of the current invention over the previouslycited patent is the capability to optimize the flux density at thetarget for targets of varying size. Whereas larger targets of similardimensions to the source are positioned for maximum flux at or near thecircle of least confusion, as defined in U.S. Pat. No. 4,757,431,smaller targets may not be. For example, in a specific case of atoroidal reflector (r_(1x)=50 mm, r_(1y)=51.9 mm) used to concentratemaximum energy at a fiber optic target, substantial differences in thelocation of the fiber occur depending on fiber diameter and NA. When thefiber-diameter (1-mm) is similar in size to the size of source and itsNA matches that of the reflector, the fiber is located near the circleof least confusion as defined by the teachings of U.S. Pat. No.4,757,431, whereas when the NA is substantially smaller, its locationfor maximum collectable flux density can vary by 0.5 mm. For two fiberseach having a diameter similar to the size of the source but differingin numerical aperture by approximately a factor of 2, the locations ofeach for which flux density is maximized differ by 1.5 mm, because theimage points of maximum brightness depend on the angular distribution offlux density which must be optimized for numerical aperture at thetarget relative to both the NA of the reflector and the brightness ofthe source. Although small differences of <50 μm in location relative tothe size of the fiber or the size of the source make no measurabledifference in the collectable flux transmitted through the target,larger dislocations clearly have a measurable impact. To find thelocation of maximum collectable flux density for a given target requiresadjustment of the off-axis toroidal mirror. Hence for unmatched opticalsystems, target and reflector, the positioning of the target may bedifferent from that of matched optical systems for a given source.

U.S. Pat. No. 4,757,431 teaches that the collecting and condensingsystem is NA independent. The present off-axis collection systemrequires that the NA's of the fiber optic target(s) and off-axisreflector(s) be matched or optimized to achieve maximum collectable fluxdensity. For unmatched systems, the reflector must have a NA that isgreater than that of the target to achieve maximum collectable flux by afiber target. For radiation emitting sources, high NA targets andreflectors provide for maximum optical and collection efficiencyresulting in maximum concentration of flux density at the target. Anoptimized system involves matching the characteristics of the source tothose of both the off-axis reflector and the target. This involves, foran arc lamp or gas discharge lamp as source, matching the brightness;i.e., intensity contour over the angular extent of the source, to theoptical characteristics of the system. For any given set ofcharacteristics describing the off-axis reflector and the target, thereis an optimum arc size and effective NA that will produce the highestbrightness source able to be coupled maximally to the target.

In the idealized embodiment of the present invention shown in FIG. 4,the source (S_(o)) and target (T) are located at focal pointsequidistant and on opposite sides of the optical axis of toroidalreflector, M₁, also defined as the system optical axis. The y-z plane issaid to contain the source, target and optical axis. In the specificcase of an arc lamp as a source (FIG. 5), an x-axis is said to beparallel to the longitudinal axis of the source, defined by theelectrodes of the arc gap. The y-z plane containing the optical axis mayor may not coincide with the plane containing the arc and fiber optictarget. At times it may be desirable to have either β≠0 or γ≠0 or both,the exact values depending on the characteristics of the arc, thereflector, optional transmissive optic(s), and the optical fiber. Inpractice, the collectable flux and system efficiency may be increased by5-10% by tilting the plane of the optical axis β≈5° and/or γ≈5° toachieve optical matching of an arc lamp source and fiber characteristicsor to locate the target above or below the y-z plane.

A more nearly ideal reduction of the present invention to practicerequires construction of a source, such as an electric arc lamp, thathouses both the off-axis mirror and retro-reflector in the sameenclosure as that containing the source. The fiber optic target may beplaced either internal or external to the enclosure. When it isinternal, the fiber is mounted permanently as a part of the fullyenclosed assembly of source, off-axis mirror, and retro-reflector. Whenit is external, either a window placed near the optimal location of afiber optic target (FIG. 8) or a fiber optic coupling mechanism is usedto couple the focused image of the source to the fiber optic target. Theperformance of such a device will depend on whether the off-axis mirrorsare toroidal or spherical and the extent to which the configuration isdisplaced off-axis. For the case in which the off-axis displacement isminimized, the performance of spherical and toroidal reflectors will besimilar. This construction eliminates aberrations that are inherent inaspheric glass envelopes associated with short arc gap lamps, and,therefore, spherical, on-axis retro-reflectors will perform as well astoroidal designs. Arc lamps constructed without aspheric glassenvelopes, such as those constructed with ceramic enclosures and awindow(s), are able to avoid envelope-induced aberrations and tosimulate near ideal conditions without enclosing the source and opticsin a single enclosure.

U.S. Pat. No. 4,757,431 teaches that the use of a spherical reflectorimposes the restriction that y_(o) ²/r<s_(o). This restriction limitsthe physical design of the system by requiring that the target beplaced, in practice, at the minimum off-axis distance, i.e., adjacent tothe source envelope. By contrast, the radii (r_(1x) & r_(1y)) of thetoroidal reflector can be chosen such that this restriction on the valueof y_(o) ²/r is considerably relaxed. This allows for additional spacebetween the source/source envelope and the target. The additional spaceeliminates potential obstruction of the focal point by the envelope andpermits the placement of optical elements (e.g., filter, correcting tiltplate, lens, etc.) or mechanical elements (e.g., shutter, iris, etc.) toattenuate, control, and/or filter the radiant flux density incident onthe target. In the preferred embodiment of the previously cited patent,to obtain the maximum collectable flux with a spherical reflector withr=50 mm and s_(o)≈1 mm, y_(o) is restricted to be no greater than ≈7 mm.Since the maximum envelope diameter is 4y_(o), which is consistent withboth the constraint that y_(o) ²/r<s_(o) and that the off-axis distancebe a minimum, the maximum diameter of the lamp envelope required by U.S.Pat. No. 4,757,431 is ≈28 mm. In practice this places the optical fiberadjacent to the envelope. Substituting a toroidal reflector withr_(1x)=50 mm and r_(1y)=51.9 mm, the optimized position for a 1 mmoptical fiber is y_(o)=10 mm and the total collected energy at the fiberoptic target is 40% greater than that achievable from the comparable 50mm diameter spherical reflector taught by U.S. Pat. No. 4,757,431. Thusy_(o) ²/r>s_(o) for the invention described here and this featureenables an optical fiber target to be placed away from the envelope.This improvement also facilitates the use of lamps with a largerdiameter. Since the diameter of an arc lamp is proportional to itsoperating wattage, a larger lamp envelope allows the arc lamp to beoperated at a higher wattage, thereby increasing the collectable flux.For the example cited here, a higher wattage lamp with a≈40 mm diameterenvelope could be used.

Since most arcs are not symmetrical and differ in their x and ydimensions, an improvement on the order of 10% in collected radiationcan be realized by polishing a cylindrical optical fiber at an anglesuch that the cross sectional area resembles an ellipse. By changing theangle of the proximal end of the fiber from normal to the optical axis,the longitudinal axis of the fiber optic target can be pivoted orswiveled to maximize the collectable flux density and the irradiation orillumination of the target.

The basic optical configuration described herein can be expanded toinclude a second concave reflector M₂ (i.e. retro-reflector). Thisretro-reflector is located behind the source to reflect and re-focusflux from and back through the source onto the toroidal reflector. Theconcave reflector can be either spherical or toroidal. The improvementin the collectable flux in using such a concave retro-reflector dependson the characteristics of the source, source envelope, off-axis toroidreflector, and target and varies from 10% to 75%. The retro-reflectorshould be optically matched to the source and its glass envelope (ifpresent) as well as to the toroidal reflector and target to produce asystem that maximizes both collectable radiation flux and systemefficiency. Toroidal designs are superior for sources having glassenvelopes (enclosures) because they facilitate the reduction ofastigmatism caused by the aspheric envelope. Correction of thisastigmatism can produce improvements in total collected radiation fluxby 20% over a spherical retroreflector. A self-contained system as shownschematically in FIG. 8 could be optimized with either a spherical ortoroidal retro-reflector depending on the target.

The optical configuration described herein can be expanded to includemultiple off-axis reflectors (as are discussed in the previously citedU.S. Pat. No. 4,757,431) multiple retro-reflectors and multiple targets.The optical system of FIG. 4, without retro-reflector M₂, couldaccommodate a total of four off-axis reflectors and four targets.Inclusion of the second reflector M₂ reduces the system to two off-axisreflectors and two targets as shown in FIG. 7. FIG. 8 shows thereduction of reflectors M₂ in FIG. 7 to a single nearly hemisphericalretro-reflector. In the case of four off-axis reflectors, each reflectorwould collect electromagnetic radiation from the source over ≈90° solidangle. In the case of two off-axis reflectors, each reflector wouldcollect over ≈90° solid angle from the source, and a pair ofretro-reflector M₂ (or the single retro-reflector as shown in FIG. 7b)would focus light back through the source over ≈90° or ≈180° solid anglerespectively. FIG. 8 shows the reduction of the optical configuration inFIG. 4 in which the two reflectors and sources are assembled andsubstantially fabricated as one self-contained unit. In practice anycombination of off-axis mirrors and retro-reflectors with a means tocouple a target to the concentrated flux density either through a windowor fiber optic fitting could be fabricated as one self-contained unit.Although the number of off-axis mirrors could be increased beyond 4 forapplications requiring more than 4 targets, in practice such an opticalsystem would not maximize the collectable flux density at the target.

In accordance with one embodiment, the present invention is directed todevices in which the square of the off-axis distance divided by theradius of curvature is equal to or less than the extent of the source ofelectromagnetic radiation (y₀ ²/r≦s₀).

What is claimed is:
 1. A system comprising: a source of highlydivergent, broadband, incoherent electromagnetic radiation, said sourcehaving an extent; an electromagnetic radiation reflector having an axisand a substantially toroidal reflecting surface concave to said source,said toroidal reflecting surface having first and second radii ofcurvature in first and second orthogonal planes, respectively, with saidfirst radius of curvature being greater than said second radius ofcurvature, said source being located near a center of curvature of saidreflector but laterally offset from the axis of said reflector by adistance, the ratio of the square of said distance to said first radiusof curvature being equal to or less than said extent; and a fiber optictarget positioned to collect electromagnetic radiation provided by saidsource and collected by and reflected from said reflector.
 2. A systemfor condensing, concentrating and collecting electromagnetic radiationto provide a high intensity illumination source having as much radiationflux as possible in a small area, said system comprising: a firstprimary electromagnetic radiation reflector having a first primaryoptical axis and a first substantially toroidal concave reflectingsurface defined by a first radius of curvature in a first plane and asecond radius of curvature in a second plane orthogonal to said firstplane, said first radius of curvature and said second radius ofcurvature of said first primary reflector being unequal; a source ofelectromagnetic radiation having an extent and located near a center ofcurvature of said first primary reflector but laterally offset in saidfirst plane by a first, off-axis distance from the first primary opticalaxis so as to produce a substantially focused image of said source, uponreflection from said first primary reflector, at a first image pointlaterally offset in said first plane from said first primary opticalaxis by a second, off-axis distance, wherein the ratio of the square ofsaid first, off-axis distance to the first radius of curvature of saidfirst primary reflector is equal to or less than the extent of thesource; and a target located near a point of maximized collectable fluxdensity, the location of said point of maximized collectable fluxdensity being a function of the first and second radii of curvature ofsaid first primary reflector.
 3. The system of claim 2, wherein saidsecond radius of curvature of the first primary reflector is selected toproduce the maximum collectable flux density for said source withrespect to said target.
 4. The system of claim 2, wherein the maximizedcollectable flux density exceeds a maximized collectable flux density ofa spherical system having a spherical reflecting surface with a singleradius of curvature but otherwise being identical to the system of claim2.
 5. The system of claim 2 wherein said first and second, off-axisdistances are selected to be greater than an off-axis distance of aspherical system having a spherical reflector and imaging the same fluxdensity at a point of maximized collectable flux density as the systemof claim 2 does, but otherwise being identical to the system of claim 2.6. The system of claim 2, wherein the difference between the first andsecond radii of curvature of said first primary reflector is minimizedand wherein the second radius of curvature of said first primaryreflector is selected to yield the maximum collectable flux densityattainable for said target.
 7. The system of claim 2, wherein saidsecond radius of curvature of the first primary reflector is selected toproduce said point of maximized collectable flux density at said targetbased on the size, brightness, numerical aperture, and opticalaberrations of said source.
 8. The system of claim 7, wherein the secondradius of curvature of said first primary reflector is selected tocorrect for aberrations of the source resulting from a source enclosure.9. The system of claim 2, wherein said second radius of curvature of thefirst primary reflector is selected to produce said point of maximizedcollectable flux density at said target based on the size, shape, andoff-axis displacement of said target.
 10. The system of claim 2, whereinsaid second radius of curvature of the first primary reflector isselected to produce an image of said source, at said target, which isapproximately the same size as said source.
 11. The system of claim 2,wherein said target is located at said point of maximized flux density,which is substantially coincident with the first image point.
 12. Thesystem of claim 2, wherein the target is an optical fiber target havinga collection end positioned near the point of maximized collectable fluxdensity.
 13. The system of claim 2, further comprising a secondaryelectromagnetic radiation reflector having a secondary optical axis anda reflecting surface defined by a first radius of curvature and asecondary second radius of curvature, said secondary reflecting surfacedisposed behind said source, with respect to said first primaryreflector, to reflect electromagnetic radiation from, and back through,said source and to produce a point of maximized flux density for saidtarget.
 14. The system of claim 13, wherein said second radius ofcurvature of said secondary reflector is selected to produce said pointof maximized flux density based on the size, brightness, numericalaperture, and optical aberrations of said source.
 15. The system ofclaim 13, wherein the second radius of curvature of said secondaryreflector is selected to produce an image of said source substantiallycoincident with said source, the image of said source beingapproximately the same size as said source.
 16. The system of claim 13,wherein the first and second radii of curvature of said secondaryreflector are equal such that the reflecting surface of the secondaryreflector is spherical.
 17. The system of claim 14, wherein the secondradius of curvature of said secondary reflector is selected to correctfor optical aberrations of the source caused by a source enclosure. 18.The system of claim 2, wherein the source of electromagnetic radiationcomprises a light source selected from the group consisting of anelectric AC arc lamp, an electric DC arc lamp, a gas-discharge lamp, afilament lamp, a light emitting diode, and a semi-conductor laser. 19.The system of claim 2, further comprising a correcting optic placedbetween said first primary reflector and said target for improving thefocus of said focused image on said target.
 20. The system of claim 2,wherein said target comprises a first optical fiber target having acollection end thereof located near the first image point, said systemfurther comprising: a second primary electromagnetic radiation reflectorhaving a second primary optical axis and a second substantially toroidalconcave reflecting surface with a center of curvature; said source ofelectromagnetic radiation comprising a light source, said light sourcebeing located near an intersection of the first and second primaryoptical axes but offset by a third, off-intersection distance from saidintersection so as to focus and concentrate the light of said source atsaid first image point and at a second image point offset from saidsecond primary optical axis by a fourth, off-axis distance, said firstimage point being opposite said light source with respect to said firstprimary optical axis and said second image point being opposite saidlight source with respect to said second primary optical axis; and asecond optical fiber target having a collection end thereof located nearthe second image point.
 21. The system of claim 20, wherein the firstand second primary reflectors are fabricated as a single reflector. 22.The system of claim 20, further comprising concave first and secondsecondary retroreflectors located generally behind the source withrespect to the first and second primary reflectors, respectively, so asto reflect light from, and back through, the source to form first andsecond images of the source substantially coincident with the source.23. The system of claim 22 wherein the light source, first and secondprimary reflectors, and first and second secondary retroreflectors arefabricated and mounted substantially as a unit within a single enclosureand coupled to said first and second optical fiber targets.
 24. A systemfor condensing, concentrating, and collecting light to provide a highintensity light source for illumination having as much radiation flux aspossible in a small area, said system comprising: a primaryelectromagnetic radiation reflector having an optical axis and a portionof a substantially toroidal reflecting surface defined by a first radiusof curvature in a first plane and a second radius of curvature in asecond plane orthogonal to said first plane, said first radius ofcurvature and said second radius of curvature being unequal; a lightsource having an extent and providing divergent, broadband, incoherentlight, said source being surrounded by an envelope and being locatednear a center of curvature of said reflector but laterally offset insaid first plane by a first, off-axis distance from the optical axis ofsaid reflector so as to produce a substantially focused image of saidsource, upon reflection from said reflector, at an image point laterallyoffset in said first plane from said optical axis by a second, off-axisdistance, wherein the ratio of the square of said first, off-axisdistance to the first radius of curvature of said primary reflector isequal to or less than the extent of the source; and, an optical fibertarget having a longitudinal axis and a collection end located near apoint of maximum collectable flux density, the location of said maximumcollectable flux density being a function of the first and second radiiof curvature of said primary reflector.
 25. The system of claim 24,wherein said first and second radii of curvature of said primaryreflector are selected to cause the position of said point of maximumcollectable flux density to be exterior to said envelope.
 26. The systemof claim 24, wherein said second radius of curvature of said primaryreflector is selected to produce the maximum collectable flux densityattainable for said source with respect to said optical fiber target.27. The system of claim 24, wherein the difference between the first andsecond radii of curvature of said primary reflector is minimized andwherein the second radius of curvature of said primary reflector isselected to produce the maximum collectable flux density attainable forsaid optical fiber target.
 28. The system of claim 24, wherein saidsecond radius of curvature of the primary reflector is selected toproduce said point of maximum collectable flux density at said opticalfiber target based on the size, brightness, numerical aperture, andoptical aberrations of the envelope of said source.
 29. The system ofclaim 24, wherein said second radius of curvature of the primaryreflector is selected to produce said point of maximum collectable fluxdensity at said target based on the size, shape, numerical aperture, andoff-axis displacement of said optical fiber target.
 30. The system ofclaim 24, wherein said optical fiber target has a numerical aperture andthe numerical aperture of said optical fiber target is not greater thanan effective numerical aperture of said source and said reflector andwherein said optical fiber target is offset from the image point of saidprimary reflector.
 31. The system of claim 24, wherein said opticalfiber target is located at said point of maximum flux density, which issubstantially coincident with the image point.
 32. The system of claim24, wherein said envelope has a longitudinal axis and wherein thelongitudinal axis of said envelope is tilted relative to a perpendicularto said first plane to increase the flux density at the target.
 33. Thesystem of claim 24, wherein said reflector is tilted at an anglerelative to said first plane to increase the flux density at the target.34. The system of claim 24, wherein the collection end of said opticalfiber target is polished at an angle selected to maximize the amount oflight collected by said optical fiber target and to correct for opticalaberrations caused by said envelope and said primary reflector.
 35. Thesystem of claim 34, wherein said collection end is perpendicular to thelongitudinal axis of said optical fiber target.
 36. The system of claim24, wherein said optical fiber target is comprised of a plurality ofoptical fibers bundled together.
 37. The system of claim 24, furthercomprising a correcting optic disposed between said primary reflectorand said optical fiber target for improving the focus of said focusedimage on said optical fiber target.
 38. The system of claim 24, furthercomprising a secondary light reflector having a secondary optical axisand a secondary reflecting surface defined by a first radius ofcurvature and a second radius of curvature, said secondary reflectingsurface being disposed behind said source, with respect to said primaryreflector, to reflect light from, and back through, said source andproduce a point of maximum flux density for said optical fiber target.39. The system of claim 38, wherein the second radius of curvature ofsaid secondary reflector is selected to produce said point of maximumflux density based on the size, brightness, numerical aperture, andenvelope aberrations of said source.
 40. The system of claim 38, whereinthe second radius of curvature of said secondary reflector is selectedto produce an image of said source substantially coincident with saidsource and being approximately the same size as said source.
 41. Thesystem of claim 38, wherein the first and second radii of curvature ofsaid secondary reflector are equal such that the secondary reflectingsurface is spherical.
 42. The system of claim 2, wherein said second,off-axis distance is substantially equal to said first, off-axisdistance.
 43. The system of claim 24, wherein said second, off-axisdistance is substantially equal to said first, off-axis distance. 44.The system of claim 16, wherein the first and second radii of curvatureof the secondary reflecting surface are unequal.
 45. The system of claim38, wherein the first and second radii of curvature of the secondaryreflecting surface are unequal.
 46. The system of claim 2, wherein themaximized collectable flux density exceeds a maximized collectable fluxdensity of a spherical system, said spherical system having a sphericalreflecting surface with a single radius of curvature, the ratio of thesquare of an off-axis distance for said spherical system to an extent ofa source for said spherical system being less than the single radius ofcurvature of said spherical system.
 47. The system of claim 2, whereinthe first radius of curvature of said first primary reflector is aneffective radius equivalent to a single radius of curvature of aspherical system, said spherical system comprising a reflector having aspherical surface.
 48. The system of claim 2, wherein the first radiusof curvature of said first primary reflector is an effective radius thatis less than a single radius of curvature of a spherical system, saidspherical system comprising a reflector having a spherical surface. 49.The system of claim 2, wherein said first radius of curvature of thefirst primary reflector is greater than the second radius of curvatureof the first primary reflector.